This invention relates to gas cooled dynamoelectric machines; and, in particular, this invention relates to a monitoring system for the early detection of localized overheating which may occur in a gas cooled dynamoelectric machine.
Localized overheating may occur within the stator core of a large gas cooled dynamoelectric machine causing damage to the machine core which may lead to extensive machine outage and costly repairs. One of the ways in which localized overheating can occur is by damage to the surface of the stator teeth such that electrical contact can take place between core laminations leading to a flow of electrical current and resistance heating when the machine is under load. Localized overheating of this nature can generate enough heat to melt the core laminations. It is therefore highly desirable to provide an apparatus for the early detection of localized overheating in a dynamoelectric machine so that machine load can be reduced and corrective action taken prior to the occurrence of severe damage.
It has been found that at the onset of machine overheating there may become entrained in the machine gas coolant certain sub-micron particles called pyrolysates which result from the decomposition of certain machine coatings at relatively safe temperatures. In U.S. Pat. No. 3,573,460 to Skala, issued Apr. 6, 1971, a device is described which will detect the presence of sub-micron particles in the machine gas coolant. The patented device comprises an ionization section having a radioactive source applied thereto and a detector section having a voltage source applied thereto. The carrier gas is ionized and a current measurement is taken across the ionized gas flowing between two oppositely charged electrodes to detect the presence of submicron particles. The electrical conductivity of the ionized gas changes if submicron particles are present in the gas carrier and hence the detected current will decrease because of the collision of ions in the gas with any submicron particles, since these ions will attach themselves to the particles and will not contribute to the ion current. The device described in the Skala patent may be generally referred to as an ion chamber detector.
The foregoing has been applied in a monitoring apparatus for a gas cooled dynamoelectric machine as is described in U.S. Pat. No. 3,427,880 to Grobel et al, issued Feb. 18, 1969. An early warning of localized overheating within a dynamoelectric machine may be obtained by monitoring the gas coolant for submicron particles using an ion chamber detector.
Within the environment of a gas cooled dynamoelectric machine, oil mist particles may become entrained in the machine gas coolant being sampled by the ion chamber detector. It has been found that such oil mist particles may also cause a decrease in the ion chamber current thereby resulting in a signal output from the ion chamber detector which is largely indistinguishable from an output signal caused by pyrolysate particles. Since the intended function of the monitoring apparatus is to detect machine overheating, the oil mist signal then becomes a false signal which may cause station personnel to shut-down a machine prematurely or alternatively allow the machine to further overheat while awaiting a validation of the signal. This problem has been largely overcome by an invention described in U.S. Pat. application Ser. No. 719,838 filed Sept. 2, 1976 and incorporated herein by reference. In that Patent Application, a heated ion chamber detector has been devised which obviates the false signal which may be caused by oil mist particles. The ion chamber detector is heated to a temperature sufficient to gasify the oil mist particles while allowing the less volatile pyrolysate submicron particles caused by localized overheating to remain unaffected by the applied heat source.